The present invention relates to a semiconductor device, display device and electronic apparatus, and more particularly, to a display device having a driving circuit provided on an active matrix substrate.
In a semiconductor device, it is important to take measure against the electro-static damages (ESD) when a high voltage caused by a static electricity or a various noise phenomenon is unintentionally applied to the inside of the device to damage an insulating film, thereby causing a fatal damage to the device. In order to prevent the problem, various protective circuits have been proposed. For example, in Patent Documents 1 and 2, it has been proposed that an abnormal voltage applied to lines is discharged to GND, etc. by proper combination of diodes. However, since general semiconductor devices are formed on a silicon wafer made of conductive material, there are little chances of being electrified within the semiconductor device, so that it is a key issue to prevent the damage when a static electricity flows through input terminals. To solve such a problem, conventionally, protective circuits are formed between input terminals and the semiconductor device forming a circuit, i.e., immediately after the input terminals, so that it is prevented that the high voltage from the input terminals is transmitted to the device.
On the other hand, in a display device using an active element such as a thin film transistor (TFT), which has been rapidly spread in recent years, a device is formed on an insulating substrate. As a result, there is a problem in that it is easy that the device is easily electrified by static electricity, and that the electro-static damage is generated during the manufacturing process. On this account, for example, an electro-static measure is taken by forming an antistatic line, called as a guide ring or a short ring, at the periphery of an active matrix circuit.
In recent years, a display device having a built-in driving circuit has been widely spread, in which an improvement of a definition, a cost reduction and better reliability are realized by using a polysilicon TFT having polysilicon serving as an active layer. In such a device, as a measure against the static electricity of the driving circuit, a conventional method used in a semiconductor on a silicon wafer is applied as it is, so that only protective circuits for preventing the static electricity from the input terminals are built in, and a conventional method of a display device not having the built-in driving circuit is used to protect the active matrix circuit from the static electricity. Hereinafter, the conventional art relating to the protection of the driving circuit will be described in detail with respect to FIG. 11.
FIG. 11 is a view illustrating a configuration of an active matrix substrate of a VGA-LCD using polysilicon TFT by the conventional art. Reference numerals 201-1 to 201-480 indicate scanning lines of the active matrix circuit, driven by a scanning line driving circuit 800. Reference numerals 202-1 to 202-1920 indicate data lines, and pixel transistors 401 composed of n-type transistors and pixel electrodes 402 are formed at intersections of the scanning lines and the data lines, respectively, to drive a liquid crystal device.
Reference numerals 801-1 to 801-480 indicate scanning line driving unit circuits composed of 480 stages, which form the scanning line driving circuit 800, and each of them is composed by a CMOS circuit. A specific configuration of the scanning line driving unit circuits 801-1 to 801-n is shown in FIG. 5.
Each of the scanning line driving unit circuits 801-1 to 801-480 is connected to a low potential power line 750 and a high potential power line 751, through which power is supplied. The low potential power line 750 is connected to a low potential power terminal 650, the high potential power line 751 is connected to a high potential power terminal 651. Also, the low potential power terminal 650 and the high potential power terminal 651 are connected to a power IC through a FPC, and they are supplied with reference potentials of a potential VS and a potential VD, respectively. Herein, VS is less than VD.
Further, a signal (for example, a clock signal) required for the operation of each of the scanning line driving unit circuits 801-1 to 801-480 are supplied from an external IC through signal lines 701 and 702 and signal terminals 601 and 602.
Here, in order to prevent TFTs constituting each of the scanning line driving unit circuits 801-1 to 801-480 from being damaged by the static electricity or noise current input through the low potential power terminal 650, the high potential power terminal 651 and the signal terminals 601 and 602, static electricity protection circuits ESD1 to ESD4 are provided between the input terminals 601, 650 and 651 and the scanning line driving unit circuits 801-1 to 801-480, respectively. A specific configuration of the static electricity protection circuit ESD1 to ESD4 is shown in Patent Document 1 and Patent Document 2.
[Patent Document 1] Japanese Patent No. 2884946
[Patent Document 2] Japanese Patent No. 3141511
However, in the driving circuit on an insulating substrate, it is difficult for charges to escape as compared to the circuit on silicon wafer, and it is easy to be electrified. In addition, in the case that the driving circuit of the display device is formed on a glass substrate, the area of the circuit is larger as compared to the IC formed on a general silicon wafer. Therefore, in such a configuration, there is a problem in that a protection of the wiring lines far from the input terminals against the static electricity is not sufficient, especially during a manufacturing process. Specifically, since there is a possibility that the static electricity is electrified on a certain location on the substrate during the manufacturing process, when the distance from the electrified location to the static electricity protection circuit is long and the line resistance is high, a transistor within a circuit located closer than the static electricity protection circuit is damaged by the static electricity even before the static electricity protection circuit starts the operation. Such a problem is the same in a semiconductor device using a SOI substrate.
Further, in a case of an insulating substrate, a large current may flow in the wiring lines by electrostatic induction when strong electrostatic discharge occurs in the outside of the substrate even in a finished product state after completion of the manufacturing process. Even in such a case, there is a possibility that a circuit located far from the terminal is damaged.
Furthermore, the present invention provides a measure against the problem in that potentials on the wiring lines are instantaneously changed, when a large amount of current flows in the wiring lines through a circuit.